David A. Rogers

Combining Biodiversity Resurveys across Regions to Advance Global Change Research

More and more ecologists have started to resurvey communities sampled in earlier decades to determine long-term shifts in community composition and infer the likely drivers of the ecological changes observed. However, to assess the relative importance of and interactions among multiple drivers, joint analyses of resurvey data from many regions spanning large environmental gradients are needed. In this article, we illustrate how combining resurvey data from multiple regions can increase the likelihood of driver orthogonality within the design and show that repeatedly surveying across multiple regions provides higher representativeness and comprehensiveness, allowing us to answer more completely a broader range of questions. We provide general guidelines to aid the implementation of multiregion resurvey databases. In so doing, we aim to encourage resurvey database development across other community types and biomes to advance global environmental change research.

Trait–environment relationships remain strong despite 50 years of trait compositional change in temperate forests

Temperate North American forest communities have changed considerably in response to logging, fragmentation, herbivory, and other global change factors. Significant changes in the structure and composition of seemingly undisturbed Wisconsin forest communities have occurred over the past 50 years, including widespread declines in alpha and beta species diversity. To investigate how shifts in species composition have affected distributions of plant functional traits, we first compiled extensive data on understory plant species traits. We then computed community-weighted trait means and functional diversity metrics for communities in both the 1950s and 2000s. We examined how trait values and diversity varied across environmental gradients and among Wisconsins four main ecoregions. Trait means and diversity values reflect conspicuous gradients in species composition, soils, and climatic conditions. Over the past 50 years, values of most traits have changed as communities shifted toward species with higher leaf nutrient levels and specific leaf area, particularly in the southern ecoregions. Trait richness and diversity have declined, particularly in historically species- and trait-rich unglaciated southwestern Wisconsin. Reductions in within-site trait diversity may be diminishing the ability of these forest communities to resist or resiliently respond to shifts in environmental conditions. Despite changes in trait and community composition, trait-environment relationships measured directly via fourth-corner analysis remain strong for most plant traits. Nevertheless, accelerating ecological change (including climate change) could outstrip the ability of plant species and traits to match their environment, particularly in more fragmented landscapes.

The pace of plant community change is accelerating in remnant prairies

Long-term legacy data reveal that the pace of plant community change in remnant prairies has accelerated. Patterns of biodiversity are changing rapidly. “Legacy studies” use historical data to document changes between past and present communities, revealing long-term trends that can often be linked to particular drivers of ecological change. However, a single pair of historical samples cannot ascertain whether rates of change are consistent or whether the impact and identity of drivers have shifted. Using data from a second resurvey of 47 Wisconsin prairie remnants, we show that the pace of community change has increased with shifts in the strength of particular drivers. Annual rates of local colonization and extinction accelerated by 129 and 214%, respectively, between 1950 and 1987 and between 1987 and 2012. Two anthropogenic drivers—patch area and fire history—increased in importance between these periods. As the strength and number of anthropogenic forces increase, rates of biodiversity change are likely to accelerate in other ecosystems as well.

Colonization and Effects of Garlic Mustard (Alliaria petiolata), European Buckthorn (Rhamnus cathartica), and Bell's Honeysuckle (Lonicera × bella) on Understory Plants After Five Decades in Southern Wisconsin Forests

Abstract Preinvasion baseline data on entire communities are absent for most taxa in most places, and this limits our ability to connect long-term ecological changes to particular invasive species or invasion events. We obtained data on forest understory composition from 94 stands in the 1950s and again the 2000s. We recorded within-stand frequency of occurrence for garlic mustard, European buckthorn, and Bells honeysuckle and identified changes in native plant species density in 20, 1-m2 quadrats in invaded and noninvaded stands. All three invasive species were absent from all study sites 50 yr ago, yet at least one was present in 77.7% of the stands by the 2000s. All three species were present in 14.9% of the stands. Garlic mustard and European buckthorn were found at 47.9% of resurveyed sites, and Bells honeysuckle was found in 40.4% of resurveyed sites. Native understory plant species density declined an average of 23.1% during the past 50 yr. Declines were not significantly different in stands with or without invasive plants. The absence of a measurable effect by invasive plant presence or frequency could be due to invasive plants being too few to have a measurable effect at the plot scale, species density being an insensitive response variable, time lags between invasions and effects, or regional factors like development pressure and fire suppression driving density declines in both invasives and native species. Nomenclature: Bells honeysuckle; Lonicera × bella Zabel (morrowii × tatarica); European buckthorn; Rhamnus cathartica L.; garlic mustard; Alliaria petiolata (Beib.) Cavara & Grande. Interpretive Summary: Garlic mustard, European buckthorn, and Bells honeysuckle are invasive plants of Eurasian origin that have established and spread in North America. All three species invade forests, but we know surprisingly little about how the richness or diversity of plant communities responds to that invasion. The absence of preinvasion plant community data from most stands contributes to our limited understanding. In this study, we acquired high-quality plant community data that was obtained in the 1950s from 94 forest stands in southern Wisconsin. None had garlic mustard, European buckthorn, or Bells honeysuckle. We revisited those stands in the 2000s and replicated the survey methods. We were able to determine what fraction of the stands was invaded since the initial survey. Garlic mustard and European buckthorn were found in 48% of all stands, whereas Bells honeysuckle was found at 40%. Across all sites, plant species richness at the plot scale declined 26%. Species richness declines did not differ between sites with and those without invasive plants, even when the frequency of the invasive plants was taken into account. The existence of baseline data from both invaded and uninvaded stands prevented us from incorrectly attributing species richness declines to invasive species effects. We caution readers against generalizing these findings beyond southern Wisconsin, and we do not claim that these invasive species have no effect on native plant communities because this was a correlative study. We instead argue that the combined effects of fire suppression and landscape fragmentation have contributed to declines in native species richness. Once established, however, garlic mustard, European buckthorn, and Bells honeysuckle create conditions that facilitate their own persistence.

Monitoring Nonnative Plant Invasions Over Fifty Years in Wisconsin Forests

Abstract Long-term ecological change is often hard to document without reliable baseline data. We obtained baseline data for and resurveyed 80 forest understory communities in Wisconsin first surveyed 50 yr ago. Sixty-two sites were in the forested region of northern Wisconsin, and 18 were in the agricultural region of southern Wisconsin. For 50 yr, the number of sites with nonnative plants increased from 4 to 62. Species richness in 1950 had little influence on nonnative species establishment by 2002, and nonnative species did not seem to influence rates of native species loss over time. Additional index words: Ecologically invasive, forest, monitoring, plant, species loss, Wisconsin.

Sixty years of community change in the prairie-savanna-forest mosaic of Wisconsin

Abstract Biodiversity loss is a global concern, and maintaining habitat complexity in naturally patchy landscapes can help retain regional diversity. A mosaic of prairie, savanna, and forest historically occurred across central North America but currently is highly fragmented due to human land conversion. It is unclear how each habitat type now contributes to regional diversity. Using legacy data, we resurveyed savanna plant communities originally surveyed in the 1950s to compare change in savannas to that in remnant forests and prairies. Savanna community structure and composition changed substantially over the past 60 years. Tree canopy density nearly doubled and many prairie and savanna specialist species were replaced by forest and non‐native species. All three habitats gained and lost many species since the 1950s, resulting in large changes in community composition from local colonizations and extinctions. Across all three habitats, regional species extinctions matched that of regional colonization resulting in no net change in regional species richness. Synthesis—Despite considerable species turnover within savannas, many species remain within the broader prairie–savanna–forest mosaic. Both regional extinctions and colonizations were high over the past 60 years, and maintaining the presence of all three community types—prairie, savanna and forest—on the landscape is critical to maintaining regional biodiversity.